Researches are positively conducted, mainly in the medical field, on imaging apparatuses that irradiate a subject with light from a light source, such as a laser, so that the light propagates in the subject and obtain information in the subject. As a type of such an imaging technique, photoacoustic tomography (PAT) has been proposed. PAT is a technique for visualizing information regarding optical properties in a living body (i.e., a subject) by irradiating the living body serving as the subject with pulsed light emitted from a light source, receiving an acoustic wave generated from a biological tissue having absorbed the light propagated and diffused in the living body, and analyzing the received acoustic wave. With this technique, biological information, such as a distribution of optical property values (hereinafter, referred to as an optical property value distribution) in the living body, particularly, a distribution of optical energy absorption densities (hereinafter, referred to as an optical energy absorption density distribution), can be obtained.
In PAT, an initial sound pressure P0 of an acoustic wave generated from a light absorber existing in a subject can be represented by the following expression.P0=Γ·μa·Φ  Expression 1,where Γ denotes a Grüneisen coefficient, which is a result of dividing the product of a thermal coefficient of volume expansion or isobaric volume expansion coefficient (β) and a square of speed of light (c) by specific heat at constant pressure (Cp). It is known that the Grüneisen coefficient Γ is substantially constant for a specific subject. μa denotes an optical absorption coefficient of the absorber, whereas Φ denotes a light quantity in a local region (i.e., a quantity of light that the absorber is irradiated with and also called the “optical fluence”).
A change in sound pressure P representing the magnitude of the acoustic wave propagating in the subject is measured with respect to time and a distribution of the initial sound pressures (hereinafter, referred to as an initial sound pressure distribution) is calculated from the measurement result. A distribution of the product of μa and Φ, i.e., the optical energy absorption density distribution, can be obtained by dividing the calculated initial sound pressure distribution by the Grüneisen coefficient Γ.
As indicated by Expression 1, in order to obtain the distribution of the optical absorption coefficients μa (hereinafter, referred to as an optical absorption coefficient distribution) from the distribution of the initial sound pressures P0 (hereinafter, referred to as an initial sound pressure distribution), a distribution of the light quantities Φ (hereinafter, referred to as a light quantity distribution) in the subject has to be determined. Given that the light propagates in the subject like a plane wave when a region sufficiently large enough for thickness of the subject is irradiated with light of a uniform quantity, the light quantity distribution Φ in the subject can be represented by the following expression.Φ=Φ0·exp(−μeff·d)  Expression 2,where μeff denotes an average effective attenuation coefficient of the subject, whereas Φ0 denotes a light quantity incoming from a light source to the subject (i.e., a light quantity on a surface of the subject). d denotes a distance between a region of the subject surface irradiated with the light emitted from the light source (i.e., a light irradiated region) and a light absorber existing in the subject. According to PTL1, a living body is irradiated with uniform light under a plurality of conditions and the average effective attenuation coefficient μeff of the subject is calculated. The light quantity distribution Φ in the subject is then calculated based on Expression 2. The light absorption coefficient distribution μa in the subject can be determined based on Expression 1 using the light quantity distribution Φ.